Method and apparatus for stretch shaping panels



APril 1961 F. c. HOFFMAN ElAL 2,980,156

METHOD AND APPARATUS FOR STRETCH SHAPING PANELS 3 Sheets-Sheet 1 Filed Oct. 15, 1956 I2 I4 I5 23 INVENTORS FREDERICK C. HOFFMAN REED B. SCOTT Ap 1961 F. c. HOFFMAN ETAL 2,980,156

METHOD AND APPARATUS FOR STRETCH SHAPING PANELS Filed Oct. 15, 1956 3 Sheets-Sheet 2 3- 42 43 39 3| 3 3s '6 3o 53 3 34 52 41 .3 a 32 IN V EN TORS 5= E FREDERICK C. HOFFMAN REED B. SCOTT Agent April 1961 F. c. HOFFMAN ETAL 2,980,156

METHOD AND APPARATUS FOR STRETCH SHAPING PANELS Filed Oct. 15, 1956 3 Sheets-Sheet 5 \Ng 59 IO 69 s 55 5 I5 INVENTORS 62 -63 FREDERICK C. HOFFMAN 15:" a REED B. SCOTT ByE Z Agent METHOD AND. APPARATUS FOR STRETCH SHAPING PANELS Filed Oct. 15, 1956, Ser. No. 616,033

7 Claims. (Cl. 153- 2) This invention relates, in general, to metal forming methods and apparatus and, more particularly, to a method and apparatus for straightening, leveling and shaping of flat stock or integrally stiffened metal panels. integrally stiffened may be defined as: a metal panel or sheet which incorporates stiffening elements called stringers or ribs, which are not attached thereto by means of fasteners but which are integral with the panel, being generated by machining or extruding the panel in on homogeneous piece.

In the aircraft industry, as well as other industries, it has been found desirable to employ integrally stiffened metal panels, for example, in the construction of airplanes, or the like. One conventional way of deriving integrally stiffened metal panels is to form a tubular extrusion having a plurality of spaced ribs radiating about the periphery of the extrusion and extending along the longitudinal axis thereof. To convert such a tubular extrusion having integrally formed ribs into a usable panel, it is required to initially open the tube and straighten the tube to form substantially a flattened ribbed panel, sometimes followed by a heat treating process to harden the flattened panel.

Difficulties have been encountered when'emp'loying the flattened ribbed panel in some applications because of the many irregularities in the metal panel which are due to effect of many stresses and strains-found in the panel or which are a direct result of the heat treating process. In this event, an additional step is required for again flattening the sheet. This can be accomplished by hand methods or by employing a conventional stretching machine. However, it has been found that ribbed panels stretched transversely to the ribs by conventional methods and apparatus produces a longitudinal bow caused by necking of the web located between adjacent ribs. Since the ribs do not neck with the web, these act as struts and bow in a concave curve.

These difliculties are due in part to the fact that conventional ribbed panel stretching machines fail to form the ribs, as well as the panel during longitudinal stretching, and furthermore, fail to place the entire panel under a compressive force substantially normal to the panel plane during the stretching operations. I It is also important in overcoming these difiiculties to properly sequence the steps comprising the stretching operation so that transverse stretching occurs prior to longitudinal stretching. p 7

Accordingly, the present invention provides an adequate method and means for removing both largean'd minute irregularities in integrally stiffened panels or"fla t" stock wherein there is provided gripper means for engaging the panel and the rib leg of each rib during longitudinal stretching, gripper means for engaging the panel A during transverse stretching, and means for placing a compressive force normal to the panel itsel f,;prior to and during the application of full ram stretching force in the longitudinal or transverse direction, which urgestateS Patent fi 2,980,156 Patente p 251.,

ice i termined surface shape of the compressive means. This compressive force is maintained during the stretching op'f erations and the elastic strains thereby induced into the panel are removed by the action ofthe longitudinal and transverse stretching means.

An important object of this invention is to pr'ovide a sectional gripping means for a ribbed panel stretching machine comprising a plurality of cooperating segments each of which adjusts itself in cooperation with the others to grip the panel webs and rib legs positively and with substantially uniform pressure, despite surface irregularity encountered, whereby uniform and even distribution of the tensile forces may be imparted to the panel and rib legs being stretched longitudinally. I

It should be understood that the present invention is not restricted in its application to the character or size of the ribbed panels; therefore, additional objects and features will be seen from a reading of the following specification with reference to the accompanying drawf ings, wherein: I

Figure 1 is an end view of a tubular extrusion having a plurality of stiffening ribs integrally formed about the exterior periphery of the tube;

Figure 2 is an end view of the tubular extrusion of Figure 1 showing the tube being opened by a powerbrake machine;

Figure 3 is an end view of the tubular extrusion of Figure 2 showing the tube opened to' assume the shape of a semi-flattened ribbed panel; 7 b

Figure 4 is a perspective view of the basic mechanical apparatus employed for transverse ribbed panel stretch ing omitting some of its structural details;

Figure 5 is a vertical sectional view of the basic mechanical apparatus employed for longitudinal ribbed panel stretching omitting some of its structural details;

Figure 6 is a fragmentary horizontal cross-sectional view of the longitudinal stretch gripping means employed in apparatus of Figure 5; I f

Figure 7 is a vertical sectional view showing means for stretching the ribbed panel and the longitudinal stretch gripping means as taken in the direction of arrows 7-7 of Figure 6;

Figure 8 is a fragmentary vertical sectional, view of the longitudinal gripping means taken in the direction of .arrows 8-8 of Figure 6 illustrating cooperating sectional adjusting wedges for engaging the panel webs and rib legs of each rib; and r I Figure 9 is a vertical sectional view of the grippin means employed in the apparatus of Figure 4 for transverse stretching. I

Referring to Figure 1, a tubular extrusion is shown comprising a circular panel 10 having a plurality of substantially equi-spaced ribs, such as rib 11, and a gripping node 12, all of which radiate outwardly above the periphery of the circular panel. The gripping node'and the plurality of ribs extend parallel to the longitudinal axis of the tubular extrusion and are integrally formed with the circular panel during the extruding process which provides desirable stiflening and strength character istics to the circular panel. The gripping node is rather stubby compared with any of the ribs, such as rib 11 which comprises a rib leg 13 and a thickenedrib cap In order to open the extruded tube preparatory to flattening operations, a conventional table saw (not shown) may be employed for slitting the circular panel substanas shown in Figure 2.

One means for opening the tubular extrusion is show'n in Figure 2 in which a powerbrake m'achine tool is employed to progressively shape the circular panel into a panel irregularities to conform elastically to are prede semi-flat ribbed panel.- In general, the powerbrake ina- ,of the ribbed panel.

2,980,156 1 a v i chine tool comprises a base 17 carrying a die 18 and a punch 20 carried by a support 21. Die 18 is provided with at least one suitable cut or slit 22 to accommodate a' rib so that panel 10 will rest on the upper surface of the die in a position to receive the punch when force is applied in the direction of arrow 23. The engagement of the punch with the inner surface of circular panel 10 while the outer surface engages die 18 causes the panel to bend slightly adjacent each side of the rib. This action, repeated for every rib, when the die is provided with a single cut or slit shapes the panel to a semi-flattened condition as shown in Figure 3. Although many irregularities appear when the ribbed panel is viewed transversely, the panel remains substantially fiat longitudinally. Subsequent to being formedto a semi-flattened condition by the powerbrake machine tool, the ribbed panel (if formed of an aluminum alloy or the like) is sometimes subjected to a suitable solution heat treating process in order to harden the ribbed panel. It has been found desirable to heat treat and quench the ribbed panel in-a horizontal position with the ribs down. Distortion of the ribbed panel caused by heat treating is more severe when quenched in the vertical position. Preferably, the heat treated ribbed panel should be kept in a refrigerated box until the first or transverse forming operation and returned to the refrigerated box until ready for the .second or longitudinal forming operation. Refrigeration of the panel is not necessary when the longitudinal and transverse stretching operations are performed simultaneously.

Although the forming operations maybe achieved by employing a variety of mechanical, hydraulic and/or by other suitable presses, the present invention will be described with reference to a conventional double-action hydraulic press including hydro-cushioning. A portion of such a press is shown in Figures 4, and 7 wherein a base 25 is provided having a cushion or pressure member 26 movably supported by a plurality of rods 27 slidable within bores 28 provided in the base, and wherein a ram 29 isemployed having a cushion 30 carried by a plurality of rods, such as rod 31. Reciprocating cushions 26 and 30 are employed for applying a compressive force to the ribbed panel prior to and during the transverse .or longitudinal forming operations. The rods 27 and 31 are actuated in the direction of the arrows 19 to cause the. cushions 26 and 30 to apply the compressive force.

; The ribbed panel 10 is arranged in a die assembly as shown in Figure 4, the assembly comprising a frame 32 mounted on base 25 so that cushion 26 may move within the interior of the frame to engage the underside Carried on each end of the frame 32 there is a mounting member 34 and a support member 35 extending in a spaced parallel relationship with each other from one side of the frame to its opposite side;

The mounting member 34 and the support member 35 are separated by a space 36 coincident or aligned with a space 37 between member 33 and the end of the frame 32. A pair of jaw holding plates or die elements 38 are carried by mounting member 34 associated with each end of the die assembly and attachment is made to the latter member by suitable means such as bolts 39. Each die element 38 is provided with aplurality of vertical apertures 40 equally spaced apart longitudinally across the die element. The apertures are arranged on the die element so that they are aligned with spaces 36 and 37 between the mounting member 34, support member .33 and the end of frame 32 as seen more clearly in Figure 5.

Each aperture 40 i provided with a rub block 4:. attached to the die element 38 by means of bolts 42 passing through a lip 43 connecting the plurality of blocks and maintaining them in fixed spaced relationship with respect to each other. Each block is preferably composed of bronze to reduce friction and is provided with an angular cam surface 44 facing into aperture 40. The rub blocks are of suitable thickness to control panel elongation along d the panel plane and may be carried by a common lip 43 as shown in Figure 4.

The plurality of apertures 46 is adapted to receive a plurality of wedges carried by a wedge frame 46. Each wedge is provided with an angular cam surface 47 which engages the cam surface 44 of block 41 upon application of force to the ram 29 (in the direction of arrows 48) which urges the frame 46 downwardly. The wedge frame 46 is attached to the ram by means of bolts 50 extending through the frame into ram 29 said wedges 45 and ram providing activating means for said jaw holding plates. The frame is constructed so that cushion 30 carried by rods 31 may be forced downward from the frame 46 to engage a resilient pad 51, composed of rubber, placed on top of the ribbed panel assembly.

The ribbed panel 10 assembly has different characteristics for transverse and longitudinal stretching oper ations respectively since different stresses and strains are placed on the ribbed panel and should be compensated for. For example, during transverse stretching, only the main web of the panel 10' is stretched while the ribs 11 are maintained in an upright condition, but for longitudinal stretching, the ribs are stretched as well as the panel.

To accomplish transverse stretching of the ribbed panel the tool assembly as shown in Figures 4 and 9, comprises a plurality of filler bars such as bar- 52 positioned between adjacent ribs and a set of jaws 53 associated with each end of the panel carried by each jaw holding plate or die element 38 as shown in Figure -9 to act as gripping means. Each set of jaws comprises an upper jaw member 54 and a smooth surfaced lower jaw member 55 separated by the die element 38. Several bores 56 are extended through the upper jaw, die element 38 and the lower jaw 55 in alignment so that a plurality of pins such as pin 57 may be inserted therethrough to couple the set of jaws to die element 38. A key '53 may be employed to retain the pins in position.

Each of bars 52 is provided with a lip 59 which fits beneath the related rib cap 14 so that force applied to the top of the bars will be distributed across the panel web between adjacent ribs. To insure that the ribs are held upright during forming and to evenly distribute the compressive force applied by cushions 26 and 30, a resilient pad 51 composed of rubber or the like is provided between the cushion 30 surface and the plurality of bars. Disposed between each bar and the surface of the panel web, there is provided a resilient pad 60 for carrying each bar and an elongated metal sheet pressure member 61 adjacent the panel web. The combined arrangement of bar, pad and metal sheet extends along the full length of the ribbed panel and causes the bars 52 to slightly protrude above the rib caps 14 so that no downward force canbe applied directly to the ribs. A metal member 62 is carried by the lower jaw member 55 on which one end of the semi-flattened ribbed panel is placed so that node 15, for example, is in a spaced parallel relationship with the set of jaws 53. Member 62 is attached to the lower jaw member by means of a multiplicity of bolts such as bolt 63 and is provided with a serrated surface 69 engageable with the bottom surface of the ribbed panel.

For transverse stretching of panel 10, upper jaw member 54 of each set is provided with an extension 65 integral therewith which engages with a spacer bar or block element 66 placed on the ribbed panel adjacent the length of node 15. A similar bar is employed on the other end of the ribbed panel adjacent node 16. This feature of construction insures complete stretching of the ribbed panel from node 15 to node 16 and uniform transverse elongation at any point in the length of the panel and thereby permits the panel to be formed to closer tolerances than by the employment of conventional jaws and clamping arrangements.

Construction in theabove manner converts the downward force applied by ram 29 to the plurality of wedges into a lateral force by means of the engagement of angled cam surfaces 44 and 47 which causes lateral displacement of the die element and jaw sets. Since the cam surfaces carried by the wedges associated with one end of the die assembly face in an opposite direction to the cam surfaces carried by wedges associated with the other end of the die assembly, the die elements 38 are urged outwardly in opposite directions. Inasmuch as the jaw sets 53 engaging the ribbed panel are carried by the pair of said die elements 38, panel is pulled or tensioned according to the lateral displacement of the die elements.

To accomplish longitudinal stretching, the ribpanel die or tooling assembly, as shown in Figures 5, 6, 7 and 8 comprises a plurality of longitudinally extending bars, resilient pads, metal sheets and a pair of jaws associated at each end similar to the ones employed for transverse stretching. However, an upper jaw member 68 of each jaw pair is substantially the same as upper jaw member 54 employed for transverse stretching having an integral extension 70 which accommodates a plurality of adjustable wedge elements 71 disposed between adjacent ribs near each end of the panel. It has been found desirable to provide a plurality of serrations 69 on the surface of the lower jaw member for longitudinal stretching of the panel 10. Each of the adjustable wedge block elements 71 includes a first wedge block 72 which is supplemented by two additional complementary wedge blocks 73. Wedge blocks 73 are arranged to engage the panel web between the ribs 11. The blocks 72 and 73 are held together by means of a plate 74 attached to wedge block 72 by screws 75. Plates 74 are provided with elongated holes 76 through which the securing means for one of the blocks 73 passes, the other block being rigidly connected to the plate 74 by a similar securing means. Surfaces 77 of blocks 73 are provided with a plurality of serrations in order to grip the rib surfaces upon downward force being applied to block 72. 'The adjustable wedge elements are connected by wedge block 72 to a plate 78 by means of a plurality of bolts 80 for con-' venience in handling and set up of the ribbed panel assembly. 1

Means are shown in Figure 7 which are responsive to the lateral panel displacement for restricting application of force applied by the ram 29 via the plurality of wedges comprising a spring loaded rod 81 carried by the die element 38 and displaced therewith. The rod is movably supported by a frame 82 carried We support 83 attached to frame 32. Placed in the path of rod 81 travel, is an electrical switch 84 also carried on support 83. The switch is electro-mechanically associated with the power source (not shown) of the ram so that switch actuation by the rod will restrict ram action downward and consequently halt lateral displacement (stretching) of the panel.

The method of the invention as practiced by the described typical form of apparatus may be said to comprise the steps of initially splitting and partially flattening the tubular extrusion as shown in Figures 2 and 3 followed by mounting the resultant semi-flattened panel 10 on the frame 32in such a fashion that the panel edges are engaged by opposing sets of jaws 53 carried by die elements 38, respectively, as shown in Figures 4 and 9.

The bottom of panel 10 rests upon cushion 26 associated with the hydraulic press and which travels interiorly of frame 32. For transverse stretching, the opposing edges of the panel carrying nodes and 16 rest upon lower jaw member 55. Upper jaw member 54 of each set of jaws 53 may be coupled to lower jaw member 55 and both members are coupled to the movable die ele'ment'38 by the insertion of pins 57 through the common bores 56 so that extension 65 may communicate with nodes ,15 and 16 via spacer bars 66. 1

Metal sheets 61 may be inserted between adjacent ribs 11 on top of the panel webs on which may be placed pad 60 and filler bar 52. The plurality of filler bars and jaw sets may be covered with resilient pad 51.

Once the assembly has been set up and the panel properly disposed in relation to the jaw sets and cushion 26-, cushions 26 and 30 may be actuated in the direction of arrows 19 to press upon the bottom of panel 10 and the top of pad 51 covering the plurality of ribsand jaw sets. The convergence of the cushions places a corn pressive force upon the panel which presses out panel irregularities and holds the panel fiat in accordance with the flat surfaces of the cushions. It is obvious that the cushion surfaces may take any desired shape or contour and that the panel will conform with this desired shape or contour under compression.

Convergence of the cushions also actuat'es the jaw sets to clamp to the panel edges. It has been found that a cushion pressure of 25 tons, when deducted from a total clamping force of 140 tons, for example, leaves tons pressure on both jaw sets or 57 /2 tonson each jaw set. This applied compressive cushion pressu're is sufficient to properly close the upper and lower jaws 54 and 55 to grip the panel edges for both transverseand longitudinal stretch operations. In order to prevent seizing of any bearing faces, it may be convenient to employ a dry lubricant on such faces such as molybdenum 'disulphide.

Following the placing of a compressive force upon the panel, ram 29 of the hydraulic press may be actuated in the direction of arrows 48. As the ram advances towards the die assembly, the plurality of wedges 45 are received by mating apertures 40 in a fashion which causes the engagement of cam surfaces 47 provided on each wedge with the cam surfaces 44 located on the rub blocks 41. I

As the ram moves downward advancing. the wedges through apertures 40, the cam surface engagement causes the die element 38 associated with each end of the panel to move in opposing directions in substantially the same plane as the panel. In this manner vertical movement of the ram and wedges is converted into lateral displacement of the die elements and jaw sets to horizontally elongate the panel.

Upon deactivation of the ram and cushions and removal of the panel from the die assembly, inspection will find a panel substantially flattened in its transverse di rection, with respect to the longitudinally extending ribs.

For longitudinal stretching of the panel and ribs, the opposing edges of the panel are placed on the lower jaw members 55 in a fashion similar to that described above for transverse stretching, excepting that adjusting wedge elements 71 are inserted between adjacent ribs on the opposing ends which are engaged by extension 70 of upper jaw member 68. The wedge blocks 72 and 73 associated with each adjustable wedge element cooperate to engage the sides of the ribs and the panel web. Upon application of compressive force via the convergence of cushions 26 and 30 to the upper jaw member 68, the wedge blocks 72 and 73 firmly grip. the ribs and panel and the jaw sets can be said to be actuated.

Following the placing of the compressive force upon the panel and while the panel is maintained under this compressive force, ram 29 of the hydraulic press may be advanced in the direction of arrows 48. Advancement of the ram causes wedges to pass through apertures 40 forcing cam surfaces 44 and 47 to slidably engage. As in the above described transverse stretching operation, engagement of the angular cam surfaces urges opposing die elements 38 associated with each end of frame 32 to move outwardly carrying the members 71, 55 to elongate the panel and plurality of ribs in their longitudinal direction while the panel is maintained under compressron. ,1

In stretching the ribbed panel, the action of the members 71, 55 pulls the ribbed panel beyond the elastic yield point of the panel and ribs. The distance past the yield point may be readily controlled by sensing the advancement of hydraulic press ram 29 and the Wedges so that the power source for the ram may be deactivated at a predetermined point of Wedge advancement. This sensing function and automatic ram power shutofif is accomplished by the displacement of rod 81 which follows the movement of die element 38. Inasmuch as electrical switch 84 is located in the path of rod 81 travel, its activation will occur upon engagement therewith and ram power will be terminated, which in turn, halts further stretching of the ribbed panel.

Having described only a typical form and procedure of the invention we do not wish to be limited to the specific details herein set forth, but Wish to reserve to ourselves any variations or modifications that may appear to those skilled in the art and fall within the scope of the following claims.

We claim:

1. A panel forming apparatus for shaping a panel, comprising a cushion for supporting said panel, gripping means slidably affixed to said apparatus for engagement of opposite edge portions of said panel, upper pressure members selectively engageable with said panel for compressing substantially the entire plane surfaces of both sides of said panel against said cushion, activating means carried by said apparatus for reciprocating said gripping means in opposite directions for stretching said panel, a reciprocating means for applying pressure to said upper pressure members and for forcibly closing said gripping means simultaneously prior to said activating means reciprocating said gripping means.

2. A panel forming apparatus for shaping a panel, said panel having a plurality of integral stiffening ribs comprising upper and lower pressure members selectively engageable with said panel, said members contacting substantially the entire plane surfaces of both sides of said panel for compressing said panel, gripping means for gripping opposite edge portions of said panel, said gripping means comprising a pair of upper and lower jaw members, and upwardly facing cam blocks rigidly attached to each of said pair of jaw members; activating means reciprocably positioned above said pressure members and said gripping means for contacting said cam blocks to reciprocate said pair of jaw members in opposite directions, a reciprocating means forcibly closing said jaw members and applying pressure to said upper pressure members simultaneously prior to said activating .means reciprocating each of said pair of jaw members.

3. A panel forming apparatus for shaping a panel, said ,panel having a plurality of integral stiffening ribs, comprising, upper and lower pressure members selectively engageable with said panel, said members contacting substantially the entire plane surfaces of both sides of said panel for compressing said panel, gripping means for gripping opposite edge portions of said panel, said gripping means comprising a pair of upper and lower jaw members, a pair of jaw holding plates connected to each of said pair of jaw members at the outer edge portions thereof, and upwardly facing cam blocks rigidly attached to each of said plates at the outer edge portions thereof; wedge members attached to an upper reciprocable ram, said wedge memberscontacting said cam blocks for reciprocating said jaw holding plates and said jaw members in opposite directions, reciprocating means forcibly closing said jaw members and applying pressure upon said -upper pressure members simultaneously prior to said activating means reciprocating each of said pair of jaw members in opposite directions.

4. A panel forming apparatus for shaping a panel, said panel having aplurality of integral stilfening ribs, comprising a reciprocable ram member mounted abovea base, upper and lower pressure members selectively en gageable with said panel, said lower pressure member reciprocable in said base, said pressure members contacting substantially the entire plane surfaces of both sides of said panelfor compressing said panel, gripping means for gripping opposite edge portions of said panel, said gripping means comprising a pair of upper and lower jaw members, and upwardly facing cam blocks rigidly attached to each of said pair of jaw members at the outer edge portions thereof; wedge members attached to said ram member, said wedge members contacting said cam blocks for reciprocating said pair of jaw members in opposite directions, and a reciprocating means forcibly closing said jaw members and applying pressure upon said upper pressure members simultaneously prior to said wedge members reciprocating said gripping means.

5. A panelforming apparatus for shaping a panel, said panel having a plurality of longitudinally disposed ribs comprising, upper and lower pressure members selectively engageable with said panel, said members contacting substantially the entire plane surfaces of both sides of said panel for compressing said panel, gripping means for gripping opposite edge portions of said panel and said ribs for stretching said panel and said ribs in a direction parallel with the plane of said panel, said gripping means comprising a pair of upper and lower 'jaw members, a plurality of adjustable wedge elements disposed under said upper jaw members and between adjacent ribs, said wedge elements contacting said ribs and said panel upon the application of pressure to said wedge elements through said upper jaw members, and upwardly facing cam blocks rigidly attached to each of said pair of jaw members at the outer edge portions thereof; activating means positioned above said gripping means and said pressure members for contacting said cam blocks for reciprocating each of said pair of said jaw members in opposite directions, a reciprocating means forcibly closing said jaw members, said adjustable wedge elements and exerting pressure on said upper pressure members simultaneously prior to said activating means reciprocating said gripping means.

6. A panel forming apparatus as defined in claim 5 wherein, each of said adjustable wedge elements comprise a first wedge block positioned out of contact with said panel, two additional complementary wedge blocks attached to each of said adjustable wedge blocks and in direct contact with said panel and said ribs, one complementary wedge block rigidly affixed to said first wedge block and the second complementary wedge block slidably afiixed to said first wedge block, and said adjustable wedge elements positioned between adjacent longitudinal ribs at the edge portions thereof, said first wedge block positioned above said two additional complementary wedge blocks and in contact with said upper jaw members whereby said second complementary wedge block slides into engagement with said panel and ribs upon the appli cation of pressure to said wedge elements through said upper jaw members.

7. A panel forming apparatus as defined in claim 1 wherein, said apparatus includes a power interrupting switch means operable by contact with said reciprocating gripping means to limit the movement of said activating means.

References Cited in the file of this patent UNITED STATES PATENTS Watter Feb. 15, 1944 (Other references on following page) Watter May 18, 1948 Davie Jan. 11, 1949 Sutowski Sept. 22, 1953 Kay Nov. 17, 1953 Hofiman Feb. 16, 1954 Cozzo Oct. 26, 1954 Read Sept. 6, 1955 Corral Dec. 13, 1955 Peterson Oct. 23, 1956 FOREIGN PATENTS Great Britain Apr. 5, 1938 Germany Sept. 20, 1941 Australia Sept. 12, 1946 

